Extraocular muscles in the lamprey, Lampetra fluviatils L. II. Motor end plates

Motor innervation and particularly the structure of motor end plates (MEPs) was studied in the extraocular muscles of the lamprey, Lampetra fluviatilis L., by light and electron microscopy. Each muscle is supplied with numerous thin motor nerve fibres. Motor end plates are located at their ends or along their course. Two motor end plate types were distinguished: en grappe-like plates with a low acetylcholinesterase (AChE) activity were observed on thin muscle fibres, whilst en plaque-like plates with a high AChE activity were found on thick mitochondria-rich and thick multifibrillar muscle fibres. The postsynaptic membrane of the former MEP type does not show the presence of infoldings, MEPs located on thick mitochondria-rich fibres show occasional infoldings, whereas the postsynaptic membrane of MEPs present on thick multifibrillar fibres reveals numerous infoldings. Motor end plates present in the extraocular muscles in the lamprey possess features typical for higher vertebrates and elasmobranch fishes, as well as for Tunicata.     

Morphology of motor end-plate and its size relation to the muscle fibre size in the amphibian submandibular muscle.

The morphology of neuromuscular junction in the amphibian submandibular muscle has been examined both light and electron microscopically. Despite the differentiation of muscle fibres into types, both the structure and ultrastructure of the motor end plates was found fairly uniform in the examined species. The strongly positive correlation between the motor end plate surface area and muscle fibre diameter was found in Triturus and Rana. Some aspects of the motor end plate morphology are discussed in connection with the results of the author's physiological and morphological observations obtained formerly.     

Electron microscopic and histochemical observations on different types of nerve endings in the extraocular muscles of the rat

Summary Nerve endings in the extraocular muscles of the rat were submitted to histochemical tests for formalin-induced fluorescence and carboxylic esterases. Acetylthiocholine, butyrylthiocholine and -naphthyl acetate were used as substrates and iso-OMPA, 284C51, eserine and E-600 as inhibitors. The ultrastructure of the endings was studied with the electron microscope.Both single and multiple nerve terminals were observed in all six extraocular muscles. The single terminals of myelinated axons were comparable in their light and electron microscopic structure with the typical motor end plates of other striated muscles, and like these they exhibit acetylcholinesterase (AChE), non-specific cholinesterase (ns. ChE) and non-specific esterase (ns. E) activity. These endings were apposed to twitch-type muscle fibres.The multiple terminals were classified with the light microscope into two types. The larger type was 1/3 of the size of the motor end plate; 2–5 endings innervated the same muscle fibre; subneural infoldings were weakly developed and possessed only slight AChE and ns. ChE and probably no ns. E activity. No subneural lamellae were visible under the light microscope in the smaller type, which also possessed AChE and ns. ChE and was composed of 10–20 small dots dispersed along a single muscle fibre. The Schwann cells along nerve fibres leading to these two types of multiple endings exhibited ns. ChE but not AChE and ns. E activity.The ultrastructure of the two types of multiple endings was principally similar. The main difference, compared with the motor end plate, was that these endings were derived from unmyelinated axons which either make synaptic contacts along their course with the muscle fibre at variable distances (smaller-type) or these terminals were grouped closely together (larger-type).A few dense-core vesicles were observed in these unmyelinated nerves and in their terminals which were considerably smaller than those in the motor end plate. They were not always separated from each other by sarcoplasm and teloglia (larger-type) and contained also empty vesicles. The secondary synaptic clefts were often sparse and irregular or even absent, but the typical myoneural postsynaptic electron density was always observed. These multiple endings, in contrast to the motor end plate, were apposed only to muscle fibres with slow contraction.No catecholamine containing nerve endings were observed in the extraocular muscles. These observations indicate that the rat extraocular muscles have a double cholinergic innervation.The author wishes to express his gratitude to Professor Antti Telkkä, M. D., Head of the Electron Microscope Laboratory, University of Helsinki, for permission to avail himself of the electron microscope facilities.     

Analysis of miniature potentials in reinnervated rat muscle]

Miniature endplate potentials (MEPPs) are regarded as the expression of release of a single quantum of acetylcholine by motor nerve endings in the muscle. Mepp frequency is dependent on the presynaptic mechanism, but MEPP amplitudes and time courses are the result of the characteristics of pre- and postsynaptic structures and of the interaction between them. After post-traumatic reinnervation of skeletal muscles, MEPP frequency increases, reaching slowly normal values. Two groups of male, Sprague Dawley rats were used: in the first group left sciatic nerve was crushed and nerve fibres were allowed to regenerate, whereas the others were regarded as controls. MEPPs were intracellularly recorded in end plates of normal and reinnervated left extensor digitorum longus muscle. MEPPs were sampled and recorded on a personal computer, and, subsequently, amplitude, rise time and half decay time were computed. At early stage after reinnervation, MEPPs showed rise times and decay times longer than normal. Afterwards, we did not find differences between mepp time courses by normal and reinnervated end plates. The possible relationships between the results and changes in acetylcholine receptor number and type, and in acetylcholinesterase activity occurring during denervation and reinnervation are discussed.     

Quantitative electron-microscopy analysis of cranial nerves III, IV, and VI and of the extrinsic eye muscles in fog.

A comparative study of the quantitative data of the frog extraocular muscles and the cranial nerves that innervate them was performed. Oculorotatory muscles contain muscle fibres of at least 4 types which are arranged in heterogeneous layers. The zonal arrangement of the muscles does not occur on the cross-sections in the vicinity of muscle insertions. In these regions only two muscle fibre types are present and the total number of fibres is smaller by 70% than in the central region of the muscle. Most numerous are muscle fibres in the rectus inferior muscle, while the smallest number of fibres is found in rectus interior muscle. Three distinct types of nerve fibres are distinguished according to the following criteria: occurrence and thickness of myelin sheath, fibre diameter and ratio "g". The fibres with thin myelin sheaths indicate small diameters (1-5--6- mum) and their ratio "g" equals 0-82 +/- 0-08. They constitute about 30% of the myelinated fibres in the nerve supply of the oculorotatory muscles and about 14% in the supply of the retractor bulbi muscle. Both the value of the ratio "g" and a greater number of these fibres in the nerve supply of the muscles that contain slow contracting muscle fibres indicate that they are rather slow conducting nerve fibres. The range of the diameters of the fibres with thick myelin sheaths is greater (3-5--13-5 mum) and their "g" equals 0-66 +/- 0-06. These fibres constitute about 70% of the myelinated ones in the nerve supply of the oculorotatory muscles and 86% in the supply of the retractor bulbi muscles. The value of the ratio "g" in these fibres indicates that they are fast contracting ones. The smallest diameters are found in the myelinated fibres (0-5--1-7 mum). These fibres occur frequently in all the examined nerves; they constitute 36--47% of the total number of all the nerve fibres. The frog extraocular muscles are characterized by an abundal nerve supply which is reflected in the low innervation ratio (1:4--1:5). On the distal cross-section of nerves the number of nerve fibres is greater than on the proximal ones. Ganglionic neurons occur sporadically around the nerves; in the nerve III synaptic contacts between two neurons were observed.     

Neuromuscular organization of avian flight muscle: architecture of single muscle fibres in muscle units of the pectoralis (pars thoracicus) of pigeon (Columba livia)

The M. pectoralis (pars thoracicus) of pigeons (Columba livia) is comprised of short muscle fibres that do not extend from muscle origin to insertion but overlap ''in-series''. Individual pectoralis motor units are limited in territory to a portion of muscle length and are comprised of either fast twitch, oxidative and glycolytic fibres (FOG) or fast twitch and glycolytic fibres (FG). FOG fibres make up 88 to 90% of the total muscle population and have a mean diameter one-half of that of the relatively large FG fibres. Here we report on the organization of individual fibres identified in six muscle units depleted of glycogen, three comprised of FOG fibres and three comprised of FG fibres. For each motor unit, fibre counts revealed unequal numbers of depleted fibres in different unit cross-sections. We traced individual fibres in one unit comprised of FOG fibres and a second comprised of FG fibres. Six fibres from a FOG unit (total length 15.45 mm) ranged from 10.11 to 11.82 mm in length and averaged (± s.d.) 10.74 ± 0.79 mm. All originated bluntly (en mass) from a fascicle near the proximal end of the muscle unit and all terminated intramuscularly. Five of these ended in a taper and one ended bluntly. Fibres coursed on average for 70% of the muscle unit length. Six fibres from a FG unit (total length 34.76 mm) ranged from 8.97 to 18.38 mm in length and averaged 15.32 ± 3.75 mm. All originated bluntly and terminated intramuscularly; one of these ended in a taper and five ended bluntly. Fibres coursed on average for 44% of the muscle unit length. Because fibres of individual muscle units do not extend the whole muscle unit territory, the effective cross-sectional area changes along the motor unit length. These non-uniformities in the distribution of fibres within a muscle unit emphasize that the functional interactions within and between motor units are complex.     

Increased neuromuscular activity causes axonal defects and muscular degeneration

Before establishing terminal synapses with their final muscle targets, migrating motor axons form en passant synaptic contacts with myotomal muscle. Whereas signaling through terminal synapses has been shown to play important roles in pre- and postsynaptic development, little is known about the function of these early en passant synaptic contacts. Here, we show that increased neuromuscular activity through en passant synaptic contacts affects pre- and postsynaptic development. We demonstrate that in zebrafish twister mutants, prolonged neuromuscular transmission causes motor axonal extension and muscular degeneration in a dose-dependent manner. Cloning of twister reveals a novel, dominant gain-of-function mutation in the muscle-specific nicotinic acetylcholine receptor alpha-subunit, CHRNA1. Moreover, electrophysiological analysis demonstrates that the mutant subunit increases synaptic decay times, thereby prolonging postsynaptic activity. We show that as the first en passant synaptic contacts form, excessive postsynaptic activity in homozygous embryos severely impedes pre- and postsynaptic development, leading to degenerative defects characteristic of the human slow-channel congenital myasthenic syndrome. By contrast, in heterozygous embryos, transient and mild increase in postsynaptic activity does not overtly affect postsynaptic morphology but causes transient axonal defects, suggesting bi-directional communication between motor axons and myotomal muscle. Together, our results provide compelling evidence that during pathfinding, myotomal muscle cells communicate extensively with extending motor axons through en passant synaptic contacts.     

Functional properties of the Golgi tendon organs

Golgi tendon organs are encapsulated mechanoreceptors present at the myo-tendinous and myo-aponeurotic junctions of mammalian skeletal muscles. Within the tendon organ capsule, the terminal branches of a large diameter afferent fibre, called Ib fibre, are intertwined with collagen bundles in continuity with tendon or aponeurosis at one end. The other end is connected with a fascicle of 5-25 muscle fibres, contributed by several motor units. The contraction of these fibres, exerting strain on the collagenous bundle and causing deformation of sensory terminals, is the adequate stimulus of the tendon organ. For this stimulus, the tendon organ has a very low threshold, so that a single fibre twitch can elicit a discharge from the receptor. A tendon organ can thus signal the contraction of a single one of the 10-15 motor units which contribute fibres to the fascicle connected with the receptor. The number of tendon organs present in a muscle, taken together with the fact that a given motor unit can activate several tendon organs, strongly suggests that the contraction of every motor unit in this muscle is monitored by at least one tendon organ. The exact nature of the information provided by tendon organs to the central nervous system remains an open question because no simple relation could be established between the discharge frequency of a receptor and the contractile forces of its activating motor units. It is known, however, that, due to their dynamic sensitivity, tendon organs are efficient in signaling rapid variations of contractile force. The dynamic parameters of muscle contraction prevail in the information carried by afferent discharges from tendons organs.     

Untersuchungen an isolierten Muskelspindeln der Ratte nach Cholinesterasedarstellung und Sudanschwarz-Färbung

Zusammenfassung An isolierten Muskelspindeln aus Lumbrical-Muskeln der Ratte, deren motorische Endigungen mit einer Cholinesterase-Technik dargestellt, deren Nerven- und Muskelfasern mit Sudanschwarz gefärbt worden waren, wurden lichtmikroskopisch folgende Befunde erhoben: Es gibt 2 Typen intrafusaler Muskelfasern: a) helle, den nuclear bag fibres entsprechende, b) dunkle, den nuclear chain fibres entsprechende.An beiden Fasertypen kommen distinkte motorische Endplatten vor, die gleichfalls in zwei Typen differenzierbar sind: a) Größere, aus locker gefügten, runden Untereinheiten bestehende; diese befinden sich nur an den hellen Fasern. Ihre Anzahl an jedem Polabschnitt kann bis zu drei betragen. b) Kleinere, kompakter gebaute motorische Endplatten; diese liegen nur an den dunklen intrafusalen Fasern, je Faserpol maximal eine.Außer den mehr in Polmitte liegenden distinkten Endplatten sieht man in den paraäquatorialen Polabschnitten regelmäßig eine zweite Art intrafusaler motorischer Nervenendigungen: die multiterminalen motorischen Endigungen. Diese an jedem Spindelpol sichtbaren Endigungen sind vorwiegend an den hellen intrafusalen Fasern lokalisiert.

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Studies on isolated muscle spindles of the rat after cholinesterase and Sudan black stainingA contribution to the morphology and distribution of motor nerve endings in the muscle spindles of mammals

Summary After staining of the motor nerve endings with a cholinesterase technique and of nerve and muscle fibers with Sudanschwarz B, single muscle spindles were isolated from lumbrical muscles of rats and examined by means of light microscopy.Two types of intrafusal muscle fibres were found: a) pale ones corresponding to nuclear bag fibres and b) dark ones corresponding to nuclear chain fibres.On both types of muscle fibres distinct motor end plates were observed; two types of endplates can be distinguished: a) Larger ones consisting of circular subunits in loose arrangement; they occur on pale muscle fibres only. Their number per muscle fibre pole may amount up to three, b) Smaller ones with more compact arrangement of their subunits occurring on dark fibres only. Their number per muscle fibre pole never exceeds one.Besides these distinct endplates with more mid-polar positions, a second kind of motor nerve ending is to be found in the juxtaequatorial region of every spindle pole: the multiterminal motor ending. These endings are located predominantly on pale intrafusal muscle fibres.An often very thin -fibre leads to every juxtaequatorial region, looses its myelin sheath and divides into several thin branches; these non-myelinated axon-branches may be observed over rather long distances, finally forming the multiterminal endings.Every distinct motor endplate receives its own — usually somewhat thicker—-fibre which looses its myelin sheath immediately before its termination at the endplate. The terminal parts of these fibres show no branching.The dark intrafusal muscle fibres receiving one motor end plate only, correspond to those intrafusal fibres of the rat which, according to other authors, show properties of twitch fibres. The pale fibres receiving multiterminal endings and multiple motor endplates, correspond to fibres which show some criteria of slow fibres.

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Herrn Prof. Dr. med. E. Horstmann zum 60. Geburtstag gewidmet     

Dedifferentiation and remodeling of motor endplates following experimental localized lesions of muscle fibers

Local anaesthetics, cardiotoxin and mechanical injuries may cause necrosis of muscle fibres while leaving the motor nerve fibres and their terminals intact. With local injuries to mouse muscles carried out by freezing or cutting we made a point of preserving both the nerve terminals and the muscle fibre portions on which these terminals were located. It was thus possible to follow the changes induced at endplates by these lesions. Within two or three days of the freezing or cutting, the muscle fibres underwent very different degrees of regression of the contractile material and T-system. The neuromuscular junctions also underwent changes, principally affecting their postsynaptic portion, in particular the folds of the subneural apparatus. After dedifferentiation of subsynaptic areas, we observed sprouting of the nerve terminal on muscle fibres which survived the amputation of one end and formed actively new myofibrils. This sprouting restored synaptic connections at the original sites, but with new structural features and correlative changes in the distribution of cholinergic receptors and cholinesterases. It is probable that after a phase of involution followed by a phase of recovery, the injured muscle fibres triggered off the nerve terminal sprouting which led to the remodelling of the endplates.     

Calcitonin gene-related peptide (CGRP)-immunoreactive sensory and motor nerves of the mammalian palate

The distribution of calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibres in the palate of rat, cat and monkey was studied using immunocytochemistry and radioimmunoassay. CGRP-containing nerve fibres were found, in all species studied, to form a rich plexus in the subepithelial and submucous layers, around excretory ducts and blood vessels. A small number of CGRP-containing nerve fibres penetrated the epithelium of the hard and soft palate, and terminated as free endings. Some CGRP-containing nerve fibres were found in the vicinity of the mucous glands. CGRP-immunoreactive motor end plates were seen in the striated muscle (tensor veli palatini) of the soft palate. Following capsaicin treatment a small depletion in CGRP-immunoreactive nerve fibres in the rat palate epithelium was noted. In contrast, CGRP immunoreactive fibres forming rich plexuses in other layers of the palate, including motor end plates, were not affected. The extractable CGRP showed no significant depletion (normal animals [n = 10] 21.7 +/- 2.4 pmol/g compared with capsaicin-treated animals [n = 10] 17.5 +/- 1.8 pmol CGRP/g wet weight). The reduction in the number of visible immunoreactive nerves following capsaicin application tends to confirm the sensory character of the CGRP-containing nerve fibres terminating in the epithelium of the hard and soft palate. The capsaicin insensitive CGRP-immunoreactive nerve fibres may thus have a predominantly motor function.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neuromuscular spindles of several amphibia and reptiles]

The work presents data on the structure and innervation of the nerve-muscle spindles in the soleus of the lake from Rana ridibunda, Bufo bufo, turtle Testudo horsfieldi, lizzard Lacerta agilis. The animals under study were shown to have different structure and innervation of these receptors. The thickness of the spindle connective tissue capsule has certain correlation with the width of the subcapsular space. The innervation apparatus in the muscle spindles of reptiles and turtles are similar in the following: sensory nerve terminations in the equatorial area of the spindle are represented by reticulars, bushes, loops and in the lizzard there appear annulo-spirals with a small amount of coils. The character of motor nerve terminations in polar zones of intrafusal muscle fibres in the spindles of reptiles is similar to that of the extrafusal fibres: in the shape of "bayonets" and end bundles (Endbüuschel). In the muscle fibres of the turtle and lizzard in addition to typical motor plaques there occur trail endings.     

Regeneration following orthotopic transplantation of entire plantaris muscle in Sprague Dawley rats

Regeneration of rat plantaris muscle has been studied without predeneration. The original muscle fibres were completely degenerated within one week, but were completely regenerated at the end of experiment. The weight of regenerate showed 51% recovery. The breadth, length and total area of muscle and number of muscle fibres was less by 42, 37, 26 and 29% respectively, compared to control muscle at the end of experiment. The diameter of regenerated fibres was almost normalized at the end of experiment. The number and diameter of nuclei in the regenerate was more than that of control which coincides with the increase in DNA and RNA contents. The protein contents after an initial loss started increasing and continued to do so till the end of the experiment. The better regeneration of the plantaris muscle seems to be due to the presence of supporting gastrocnemius muscle.     

Form and classification of motor endings in mammalian muscle spindles

The presynaptic features of 234 motor endings supplied to cat hindlimb muscle spindles have been studied in teased, silver preparations, and the postsynaptic features of a further 27 endings have been studied in serial, 1 micron thick, transverse sections. In the presynaptic study motor endings received by the three types of intrafusal muscle fibre were compared with the endings supplied to spindles by the various functional categories of motor axon. Three forms of motor ending were found that had significantly different presynaptic features. These forms correspond closely to those previously identified in the literature as p1 (beta), p2 (dynamic gamma) and trail (static gamma). The results of the postsynaptic study showed that the degree of indentation of the intrafusal muscle fibres by motor axon terminals increases with greater distance from the primary ending, irrespective of muscle-fibre type. We conclude that the postsynaptic form of intrafusal motor endings is determined by distance from primary ending and muscle-fibre type. It is not determined by type of motor axon, and cannot be correlated with presynaptic form so as to produce a unified classification of intrafusal motor endings.     

Comparison of force and EMG measures in normal and reinnervated tibialis anterior muscles of the rat.

The relationship between motor unit force and the recorded voltage produced by activated muscle unit fibres (electromyogram, EMG) was examined in normal and reinnervated rat tibialis anterior muscles. The number, cross-sectional area, and radial distance from the recording electrode of muscle fibres in a given unit, obtained directly from a sample of glycogen-depleted motor units, were analysed in relation to the magnitude of the EMG signal produced by that unit. EMG peak to peak amplitude and area varied as approximately the square root of twitch force in both normal and reinnervated units. Furthermore, the EMG amplitude increased approximately as the total cross-sectional area of the motor unit (number of muscle fibres x the average cross-sectional area of the fibres) and inversely with approximately the square root of the distance of fibres from the recording electrodes on the surface of the muscle.     

Relationships between mitochondrial content and glycogen distribution in porcine muscle fibres.

Serial sections of longissimus dorsi and rectus femoris muscles from 15 Yorkshire breed pigs (live weights 24-46 and 49-139 kg) were stained for glycogen (PAS) and a mitochondrial enzyme (NAD tetrazolium reductase). Muscle fibres with a low mitochondrial content in both muscles were more frequently PAS-positive than fibres with a high or intermediate mitochondrial content. However, some pigs had all their muscle fibres PAS-positive while one pig with a high post-mortem muscle pH had all rectus femoris fibres PAS-negative. Relative to lighter weight pigs, longissimus dorsi muscles of heavy pigs tended to have less fibres with a high mitochondrial content and less fibres with a positive PAS reaction. Compared to longissimus dorsi muscles, rectus femoris muscles had more fibres with a high mitochondrial content and less with a positive PAS reaction. All fibres in both muscles became PAS-negative with an accompanying decrease in pH by 24 hr post-mortem. Fibres from longissimus dorsi muscles frequently had PAS-positive sarcoplasmic cores between their myofibrils. Heavy pigs tended to have larger cores (up to a mean maximum diameter of 13.4 mum), more fibres with cores, and more cores per fibre. The pigs involved exhibited no other ante- or post-mortem muscle abnormalities.     

End plate spikes in the human electromyogram. Revision of the fusimotor theory.

'End plate spike' (EPS) is a spontaneous action potential of a normal striated muscle. EPSs are found in local 'active spots' of the muscle. The prevailing hypothesis about the origin of EPSs states that when a needle electrode affects a motor nerve branch near the neuromuscular junction at the end plate zone, an increased leakage of acetylcholine to the synaptic cleft ensues. This elicits postsynaptic action potentials of the muscle fibre which can be recorded as EPSs with the same needle electrode. Thus EPSs are thought to be caused by needle injury or irritation of the motor axon. We suggest that EPSs are action potentials of intrafusal muscle fibres and that 'active spots' are in fact muscle spindles. Waveform analysis reveals three types of EPSs: small EPSs, not propagated outside the active spot either: i) with negative onset; or ii) with short positive initial deflection; and iii) large EPSs resembling propagated motor unit potentials (MUPs) but with a typical EPS firing pattern, distinctly different from that of the MUPs. Study of EPS activation in different manoeuvres associates small EPSs with intrafusal gamma motor units and large MUP-like EPSs with beta motor units.     

Spatial analysis of motor unit potentials of the rat medial gastrocnemius

The spatial analysis of the potentials of single motor units of the rat medial gastrocnemius muscle evoked by stimulation of the fibres of split ventral roots was carried out with a bipolar electrode moving in the direction perpendicular to the longitudinal axis of the muscle fibres. During this movement of the electrode a variability was observed in the time of the biphasic potential from its maximum to minimum, and in the peak-to-peak amplitude of these potentials. The potentials recorded outside the territory of the motor unit had a lower amplitude in relation to the potentials from the territory of the unit. This made localization of the motor unit on the cross-section of the muscle possible. Differences in the duration of the potential from maximal to minimal amplitude (maximum-minimum amplitude time--M-MAT) of each investigated motor unit from successive recording sites reflected the number of fibres contributing to the action potential and the distance of the recording surface of the electrode from the zone of the motor end-plates of this motor unit. The greatest diameter of the territory of the observed motor units reached 2.5 mm.     

Succinic dehydrogenase and cholinesterase activities in the skeletal muscles of the desert rat and the albino rat

The deltoid and gluteus maximus of the desert rat and the albino rat were examined histochemically for the distribution of succinic dehydrogenase (SDH) and cholinesterase (ChE). SDH activity showed that the deltoid and gluteus maximus muscles of the two animals consisted of three types of muscle fibres, with a predominance of muscle fibres that have higher SDH activity in both the deltoid and gluteus maximus muscles of the desert rat than in the albino rat. The mean diameter of all muscle fibres in the deltoid and gluteus maximus muscles and their ratios to the average body weight were determined in both animals. The desert rat showed a statistically significant increase in those ratios compared with the corresponding ratios for the albino rat. ChE activity showed that the deltoid and gluteus maximus muscles are richly innervated by intensely positive ChE motor end-plates with a predominance of plaque-like terminals. The mean diameters of the end-plates and the ratios of these diameters to the diameter of the muscle fibres together with their ratios to the body weight were determined. A correlation between these values and the histological findings is proposed.     

The levator ani of the female rat: a suitable model for studying the effects of testosterone on the development of mammalian muscles

The present results give evidence by using cytochemical markers of motor end-plates (ChE, AchR) and ultrastructural techniques that the Levator ani (LA) muscle is also present in adult females: it is composed of differentiated and innervated fibres. A significant difference, both on the number of fibres per muscle (n) and on their average cross-sectional area (acsa), was observed between sexes:-male: n = 5300 +/- 687, acsa: 522 +/- 68.6 microns 2;-female: n = 565 +/- 246.9, acsa: 68 +/- 8.6 microns 2. These results suggest that testosterone could control, at least partially, the number and the diameter of muscle fibres during development.